Photomultiplier tubes are each capable of taking out very weak light as an electric signal, and therefore are used in various kinds of fields. For example, a photometer irradiates a sample with light to detect fluorescence generated from the sample by using a photomultiplier tube, and analyzes very small amounts of components contained in the sample. In addition, a charged particle beam apparatus irradiates a sample with an electron beam, converts a weak secondary electron generated from a surface of the sample into light by using a scintillator, and detects the light by using a photomultiplier tube to minutely observe the surface of the sample.
As a technique related to the detection of light by using such a photomultiplier tube, for example, PTL 1 (International Publication No. 2013/187511) discloses a technique related to an optical signal detection circuit provided with: amplification means for amplifying an analogue detection signal according to a light amount detected by light detection means; analog to digital conversion means for converting the analogue detection signal amplified by the amplification means into a digital detection signal; threshold value determination means for detecting a pulse from the digital detection signal converted by the analog to digital conversion means, and repeating processing of detecting the energy of the detected pulse to determine the frequency of appearance of pulses on a detected energy basis, thereby determining a pulse determination threshold value on the basis of the determined frequency of appearance of pulses on an energy basis; and threshold-value processing means for outputting, as a detection signal, the digital detection signal containing a pulse, the energy of which is higher than or equal to the pulse determination threshold value determined by the threshold value determination means.